1. Field of the Invention
The present invention relates to a radar apparatus equipped with an array antenna comprised of a plurality of antenna elements as a receiving antenna and arranged to detect a target bearing from phase differences between signals received by the respective antenna elements and, more particularly, to a method for determining phase correction values therein.
2. Related Background Art
A digital beam forming (DBF) radar apparatus is an example of the radar apparatus of this type. Since the DBF radar apparatus employs a method for electronically scanning a desired range, it needs no scanning mechanical portion and has characteristics of being compact, lightweight, and strong against vibration and the like. Because of these features, it is expected to be used as a radar apparatus for vehicle.
In the radar apparatus of this type, phases of waves reflected by a target positioned at the front or in the direction of 0.degree. must be theoretically equal among the antenna elements. Disagreement is, however, encountered between the phases because of manufacturing variations in practice. It is thus common practice to cancel out the disagreement between the phases due to the manufacturing variations by giving phase correction values to the respective antenna element channels at the stage of signal processing.
Since the phase correction values of the respective channels are specific to each radar apparatus produced, they are determined by measurement for each apparatus. As a method for determining the phase correction values there is, for example, the "Method for detecting errors of DBF radar" disclosed in Toyota Technical Reports (Toyota Gijutsu Kokaishu issue No. 7294 issued Dec. 25, 1997). According to this technique, a target is placed at a position assumed to be the front (the direction of 0.degree.) of the radar apparatus, the radar apparatus is actuated channel by channel for every antenna element, and phase correction values are determined for the respective channels so as to match the phases of receive signals at the respective channels with each other.
The phase correction values of the respective channels are stored in a memory section of the radar apparatus and correction is carried out with these phase correction values on the occasion of the DBF signal processing, thereby canceling out the phase shifts between the channels due to the manufacturing variations.
By this conventional method for determining the phase correction values, the 0.degree. directions of the respective antenna elements agree with each other to become the direction of 0.degree. in terms of the structure of the radar apparatus. There was, however, a problem that the 0.degree. direction was not always coincident with an electrical center of the radar apparatus. This is caused by assembling errors etc. between the housing of the radar apparatus and the antenna.
It raised the possibility of detection error; for example, in cases where the radar apparatus was mounted on a vehicle for the purpose of front detection, when the apparatus was mounted so as to match the 0.degree. direction of the radar apparatus with the front of the vehicle, the detection area of the radar apparatus deviated from the front of the vehicle.